For liquid crystal display devices, the classification based on the operating mode of liquid crystals includes a phase change (PC) mode, a twisted nematic (TN) mode, a super twisted nematic (STN) mode, an electrically controlled birefringence (ECB) mode, an optically compensated bend (OCB) mode, an in-plane switching (IPS) mode, a vertical alignment (VA) mode, a fringe field switching (FFS) mode, a polymer sustained alignment (PSA) mode and a field induced photo-reactive alignment (FPA) mode, etc. The classification based on the driving mode of the device includes passive matrix (PM) and active matrix (AM) types. The PM types are classified into static type, multiplex type and so forth, and the AM types are classified into thin film transistor (TFT) types, metal insulator metal (MIM) types and so forth. The TFT types are further classified into amorphous silicon and polycrystal silicon types, wherein the latter is classified into a high temperature type and a low temperature type according to the production process. The classification based on the light source includes a reflective type utilizing natural light, a transmissive type utilizing a backlight, and a transflective type utilizing both the natural light and a backlight.
The devices include a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM device having good general characteristics. Table 1 below summarizes the relationship between the general characteristics of the two aspects. The general characteristics of the composition will be further explained based on a commercially available AM device. The temperature range of the nematic phase relates to the temperature range in which the device can be used. A preferred maximum temperature of the nematic phase is about 70° C. or higher, and a preferred minimum temperature of the nematic phase is about 0° C. or lower. The viscosity of the composition relates to the response time of the device. A short response time is preferred for displaying moving images on the device. Accordingly, a small viscosity of the composition is preferred. A small viscosity at a low temperature is further preferred. The elastic constant of the composition relates to the contrast of the device. In order to increase the contrast of the device, a large elastic constant in the composition is further preferred.
TABLE 1General Characteristics of Composition and AM DeviceGeneral CharacteristicsGeneral CharacteristicsNo.of Compositionof AM Device1Wide temperature range of aWide usable temperature rangenematic phase2Small viscosity1)Short response time3Suitable optical anisotropyLarge contrast ratio4Large positive or negativeLow threshold voltage anddielectric anisotropysmall electric power consumptionLarge contrast ratio5Large specific resistanceLarge voltage holding ratio andlarge contrast ratio6High stability to ultravioletLong service lifelight and heat7Large elastic constantLarge contrast ratio andshort response time1)A liquid crystal composition can be injected into a liquid crystal cell in a shorter period of time.
The optical anisotropy of the composition relates to the contrast ratio of the device. The product (Δn×d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the device is designed so as to maximize the contrast ratio. A suitable value of the product depends on the type of the operating mode. In a device having a mode such as TN, the suitable value is about 0.45 μm. In the above case, a composition having a large optical anisotropy is preferred for a device having a small cell gap. A large dielectric anisotropy of the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio of the device. Accordingly, a large dielectric anisotropy is preferred. A large specific resistance of the composition contributes to a large voltage holding ratio, and the large voltage holding ratio contributes to a large contrast ratio of the device. Accordingly, a composition having a large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase in an initial stage is preferred. A composition having a large specific resistance at room temperature and also at a temperature close to the maximum temperature of the nematic phase even after the device has been used for a long period of time is preferred. The stability of the composition to ultraviolet light and heat relates to the service life of the liquid crystal display device. In a case where the stability is high, the device has a long service life. Such characteristics are preferred for an AM device for use in a liquid crystal projector, a liquid crystal television and so forth. A large elastic constant of the composition contributes to a large contrast ratio and a short response time of the device. Accordingly, a large elastic constant is preferred.
A composition having a positive dielectric anisotropy is used for an AM device having a TN mode. On the other hand, a composition having a negative dielectric anisotropy is used for an AM device having a VA mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having an IPS mode or a FFS mode. A composition having a positive or negative dielectric anisotropy is used for an AM device having a PSA mode or a FPA mode. A liquid crystal composition having a positive dielectric anisotropy is disclosed in Patent literature No. 1 to No. 3.